1. Field of Invention
This invention relates to a specific class of martensitic stainless steel designed and optimized for use in building mold bases for plastic injection mold tooling.
2. Description of Prior Art
Mold bases for plastic injection molds have evolved with increasing sophistication as the uses of and demands on plastics have grown. Initially these tools were constructed mainly of carbon and low alloy steels. New plastics, higher operating stresses, larger high cavitation tooling, longer production runs and inherent corrosion problems with the application led to the use of stainless steels in increasing quantities since around 1980, however the stainless steels in predominant use have many undesirable features.
The stainless steels in use for this application are essentially modifications of a standard AISI 420 stainless steel (S42000), in some cases with sulfur additions to enhance machinability. No suitable stainless grade for this specific application was ever developed. Major disadvantages of the current grades are:
(a) Very high hardenability, capable of developing Rockwell-C hardness in excess of 50 Rockwell-C. The application typically only requires hardness in the range of 32 to 39 Rockwell-C. The extreme hardenability demands extensive thermal processing at the ingot manufacture stage, hot working stage and heat treating stage to prevent catastrophic cracking in the raw product. Lengthy annealing cycles are required to render the material safe to handle and soft enough for flattening and saw cutting. Rapid change in hardness at the required tempering temperatures, resulting in non-uniformity of hardness. In brief, these steels have too much hardenability and too little ductility for the application. PA1 (b) Poor formability characteristics, making it difficult to achieve the degree of flatness desired in the product. Inherent stiffness and lack of ductility make the product unsuitable for hot or cold flattening and often leads to product breakage while attempting to flatten. PA1 (c) High residual stresses developed during hardening leads to dimensional instability during complex machining operations, and especially if large cavities are formed to accommodate large molds. PA1 (d) Poor surface quality as hot worked. This feature requires making product at oversize gauges to allow for proper surface cleanup in the final application. PA1 (e) Poor weldability, making repairs difficult and unreliable. PA1 (f) Only moderately resistant to corrosion. PA1 (g) "Breakout" at the exit surfaces when drilled. Breakout occurs when the metal surrounding the drill exit area fractures and tears away in advance of the drill, creating a ragged hole edge. PA1 (h) Extensive edge tearing during hot working, resulting in poor yields and also necessitates expensive edge trimming prior to hardening. PA1 (a) The chemical analysis of the invention simplified thermal processing of the cast ingot, resulting in cost and time savings. Ingots require only slow cooling after stripping, not a lengthy and costly furnace annealing treatment. PA1 (b) The chemical analysis of the invention is designed to yield acceptable hardness for the intended application either as hot worked, as hot worked and stress relieved, as normalized or as normalized and stress relieved. No post hot working annealing cycles are required to protect the material from cracking. PA1 (c) The inherent ductility of the invention virtually eliminates occurrences of drill breakout. PA1 (d) The chemical analysis of the invention, specifically the low carbon level, and high chromium level and addition of copper result in improved corrosion resistance. PA1 (e) The inherent ductility of the invention eliminates all occurrences of drill breakout. PA1 (f) Excellent hot worked surface quality of the invention permits a reduction in over gauge allowances necessary to meet fmal product sizes. This reduces machining and grinding costs and increases overall yield from ingot to plate product. PA1 (g) Reduced hot strength of the invention results in more effective flattening during hot working, yielding excellent as rolled flatness totally free of waviness and wrinkles. This eliminates the need for secondary cold or hot flattening operations and the improved flatness reduces the amounts of machining required to produce clean, bright metal finished surfaces. PA1 (h) High ductility of the invention provides a material which can withstand more than twice the amount of cold deformation prior to fracture compared to current modified AISI 420 grades. High ductility also allows increased levels of Sulphur without danger of hot tearing. The increased Sulphur content produces excellent machinability. PA1 (i) Calcium addition to the molten metal results in controlled sulfide morphology, eliminating objectionable stringer type sulfides. PA1 (j) Thermal conductivity of the invention has been improved by reduction of chromium and addition of copper. PA1 (k) Low carbon level and reduced hardenability result in a readily weldable material.